The present invention relates to the field of optical inspection of object contours, and more specifically, to an apparatus and method for providing front lighted, high contrast images of an object against its background.
In the art of manufacturing electronic components, it is typical to inspect the components in order to determine various features and characteristics thereof, such as shape, contour, size and the like. Such inspection is particularly useful in examination of physical vapor deposited electronic elements on printed circuit-type boards. In these devices, adherence to rigorous specifications on the size, shape, position, orientation and assembly registration may be determined by automatic image analysis of sharp, uniform shadow projections of the electronic elements and the arrays in which they are formed.
In the areas of thin film manufacture and development, contours and component areas and circuit lands are typically inspected optically by backlighted shadowgraph or profile projection techniques. Resultant sharp images permit the use of automated image analysis systems, such as computer analysis of video images, for example.
In many instances, however, the circuit to be inspected, including the circuit elements and the substrate thereof, is encased in an opaque assembled product, whether by mounting on an opaque substrate, by encapsulation, or by enclosure in an opaque container. Such opaque modules cannot be examined by the techniques of back lighting and use of transmitted light to provide a shadow, much less a clear, sharp, high contrast shadow image. Moreover, inasmuch as the circuit elements themselves are frequently specular reflectors, the use of front lighting for imaging of the objects is similarly difficult.
A further problem in attempting to view the elements, or their substrates, is the spotty nature of images produced by the elements, since typically neither the element nor the substrate forms a good mirror. Rather, both the element and substrates typically reflect and diffuse incident light unevenly, thus producing the spotty images with insufficient contrast in optical brightness and a lack of image uniformity which makes automatic image analysis of the contours and dimensions difficult.
In summary, the precision patterns of most modern electrical circuitry formed of physical vapor deposited metals on a transparent plastic substrate cannot now be inspected by transmitted back light which provides optimal profile projection. Such inspection is precluded by the opacity of the reverse-side circuitry and other stereo hindrances. Classical front lighting techniques yield reduced optical contrast between the object being viewed and its background, and provides excessive brightness variation within the image of the object due to spectral flares and shadows from surface imperfections. The object, in other words, cannot be readily resolved from its background.
In U.S. Pat. No. 3,180,216 to Osterberg, a variable phase microscope is described for enhancing the contrast of an object being examined. The disclosed apparatus, which operates by rotation of a deviated light vector to obtain destructive interference with a transmitted light vector over the image of a particle, thereby enhancing the contrast between the particle and its surround, is complex. A laser is required, and a plurality of coherent light beams are needed to illuminate the specimen. A multiapertured, opaque disk is utilized.
Prior art microscopes utilizing polarization in backside illuminated objects are known, as illustrated by the disclosure of U.S. Pat. No. 3,007,371 to Tallman. For the reasons previously described, such microscopes cannot be used for imaging opaque objects mounted on substrates.
A frontally illuminated specimen is inspected in a system disclosed in the U.S. Pat. No. 2,318,705 to Morgan. The patent teaches the use of polarizing elements, to filter reflected light from the objective of a microscope examining the specimen, in order to reduce fogging of the image. The disclosure does not, however, teach either a method or an apparatus for imaging the specimen against a bright background with frontal illumination. Rather, light reflected by an object is eliminated by a polarizing screen, while light passing through the objective to speciman 38 is passed by the polarizing screen.